1. Field of the Invention
This invention relates to aqueous artifical resin dispersions that do not contain low molecular weight emulsifying agents and more particularly to those dispersions containing a dispersed polymer having ionic or ionizable groups attached to the polymer chain.
2. Description of the Prior Art
It is known that one can largely eliminate low molecular weight emulsifiers or dispersants in the production of aqueous artificial resin dispersions if the dispersed artificial resin contains units having ionic groups, especially alkali metal sulfonates. Nevertheless, prior to the present invention no one had yet succeeded in completely eliminating low molecular weight emulsifying agents in such dispersions. In fact, small amounts of low molecular weight anionic emulsifying agents have been previously considered to be indispensable if standardization of particle size is desired. The phrase "low molecular weight" when applied to emulsifiers or dispersants of the ionic or non-ionic type in this application refers to compounds having molecular weights under 2,000.
As disclosed in German-OS No. 19 32 395, a stable lattice which does not contain a water-soluble wetting agent or surface active agent can be produced by polymerizing an aqueous emulsion of an unsaturated, largely water-insoluble monomer together with 0.1-5% by weight, based on the weight of monomer, of polymerizable organic sulfonic acids or alkali sulfonates of the formula EQU R--Z--Q--SO.sub.3 M.
In this formula R is the polymerizable portion of the monomer, Z can be an oxygen atom or an ester or amide group, Q is an aliphatic or aromatic hydrocarbon residue, and M is hydrogen or an alkali metal ion. With a sufficient size of the group Q, the compound has soap-like properties. During emulsion polymerization these compounds acts in the same manner as low molecular weight emulsifying agents. Nevertheless, the monomers become captured in the polymer as it is formed so that the sulfonic acid-containing monomer is depleted from the aqueous phase by the end of the emulsion polymerization. Although the substance acts as an emulsifying agent during the emulsion polymerization, a small amount of known wetting agents, for example, alkali soaps, are jointly used to control particle size. The low molecular weight, non-polymerizable emulsifying agents occupy, for example, 0.4% of the aqueous phase.
T. H. Mills and R. Yocum have described in Journal of Paint Technology, 39, 532-35 (1967), the production of polymer dispersions with built-in units of 2-sulfoethyl methyacrylate. Although they have also attempted to eliminate low molecular weight emulsifying agents in order to obtain low foaming dispersions and films with limited hydrophilic properties produced therefrom, they could not eliminate anionic emulsifiers completely. They obtained a dispersion with relatively coarse particles, namely 2500 A, by using 0.01 wt. %, based on monomer weight, of an anionic emulsifier. In order to obtain particle sizes of 700 A, 1.25% of a low molecular weight emulsifying agent had to used concurrently with the disclosed sulfonate-containing monomers. Thus, the goal of obtaining a dispersion free from emulsifying agents was not attained.
The polymerizible emulsifying agent as disclosed above has not proved itself suitable for controlling particle size. While the particle size decreases with increasing amounts of the customary emulsifying agents, the opposite effect has been observed for sulfoethyl methyacrylate. Based on these experiences, the customary low molecular weight emulsifying agents have been utilized concurrently with the polymerizable sulfonic acids. German-OS No. 24 27 341 discloses the amounts of these additives to be no more than 0.2 wt. % (based on monomer). German-OS No. 22-50 517 describes the addition of from 0.3 to 3% of anionic emulsifiers in the production of artificial resin dispersions, in which the resin contains polymerized units derived from monomers having the formula EQU CH.sub.2 .dbd.CH--CO--X--Y--SO.sub.3 M,
in which X is oxygen or a --NH-- group, M is an alkali metal or ammonium ion, and Y is an alkyl radical with 2-10 C-atoms. The previously described sulfonate-group-containing monomers contained C.sub.4 -groups as Y. In the absence of further emulsifying agents--so teaches this reference (column 3, lines 36-59)--a large portion of a coagulate and a dispersion that is unstable in storage arise. It is further disclosed in French-PS No. 1 536 863 that the usual emulsifying agents must be used concurrently in the presence of monomers having following formula (R=H or methyl): ##STR2##
Even though some of these publications disclose a considerable reduction in the emulsifying agent content when compared to similar artifical resin dispersions that do not contain built-in sulfonate groups, even then the detrimental effect of these emulsifying agents cannot be overlooked. As previously mentioned, the synthetic resin dispersions produced with the normal amounts of customary low molecular weight emulsifying agents produce relatively coarse particles. This is disadvantageous both for the stability of the dispersion and for the production of films from the dispersion. It is well known that emulsifying agents exert their effects even when they are present only in very minute amounts. They accelerate the penetration of water into the pores of a film made from the dispersion and thereby aid in its destruction through corrosion; this is particularly true of anionic emulsifying agents. It was the goal of the present invention to produce artificial resin dispersions which are free from low molecular weight ionic emulsifying agents and preferably also from non-ionic emulsifying agents, and which nevertheless have an optimal particle size, that is to say, one that can be selected in view of the technical application in question.